Automatic Welding Control

Weld Tube Mill*

System for Electric-resistance

By Tetsuo KYOGOKU,**

Masao TATSUWAKI****

Chiharu

and Shin

TAKAMADATE,** Kaxuyuki HOTTA,***

NEMOTO*****

Synopsis Recently, high quality Electric Resistance Welded Tubes (ERW Tubes) have been requested by the customers. To meet this demand, the Sumi-tomo Metals has successfully developed an Automatic Welding Control System, using Pattern Thermometer, Upset Gauge, etc., which were newly developed for this system. The results obtained by applying this system to a small size ER W tube mill are as follows:

(1) Welding temperature was stabilized by computer according to the conditions of the tube size, material and welding speed.

(2) This system has been found to be useful for welding low alloy steel and high carbon steel which have more severe welding conditions.

I. Introduction

Since the Sumitomo Metal Industries, Ltd. started the production of small-sized ERW tubes in 1951, it has actively involved in improving the quality control, welding techniquesl) and non-destructive testing techniques required for their production.2,3) Demands for high-reliability, high-quality and higher strength tubes have been growing since these tubes are used under increasingly more severe conditions, such as boiler tubes for modern power plants4) and for automobiles and other mechanical uses. To meet those demands, Sumitomo Metals was engaged in the development of a new automatic welding control system to establish a more reliable and efficient system than the conventional two-color pyrometer feed-back method. For this new system, two types of unique sensors have been successfully developed. One is a Pattern Thermometer which provides the welding operator with on-line information on welding tem-

peratures at 64 points in the welding zone, and the other is an Upset Gauge which is capable of con-tinuous measurement of the upset amounts. The system computer automatically makes temperature compensations for any change in wall thickness or welding speed. The computer also tracks the ends of skelps to ensure that they are cut off and rejected after welding. The system computer also prints out

production data every 5 min. This automatic welding control system ensures consistently high quality common carbon steel tubes and, makes it easy to produce low-alloy or high-carbon steel tubes using a high-frequency welder. The system has been operating successfully at No. 2 Tube Mill of the Wakayama Steel Works of Sumi-tomo Metals. The mill, which was installed in 1980,

can produce 42-in. O.D, tubes with the thickest wall (11 mm) in the world.

II. ER W Method and Prevention of Its Welding Defects

In the general process of forming high-frequency welded tubes both edges of the skelp are butted into an " I " or " V " shape. These edges are then heated by an induction coil from the corners to the middle of the wall thickness depending on high frequency current distribution (see Figs. 1(b) & (b')). Then these heated edges are squeezed together by a pair of rolls, forcing our molten metal (see Figs. 1(c) &

(c')). The following three main factors determine the weld quality: i) Welding temperature ii) Upset amount (the difference in tube circum- ference before and after welding)

iii) Edge formation. When the three conditions are met, a ferrite bands)

appears clearly at the center of the weld (see Fig. 1(c')). If they are not correct, however, defects may occur (see Table 1). Of the three factors, the edge formation is determined prior to welding, whereas

Fig. 1. Process of welding.

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Originally published in Sumitomo Metals, 35 (1983), 193, in Japanese. English version received February 17, 1984. © 1984 ISIJ Tube Making Department, Wakayama Steel Works, Sumitomo Metal Industries, Ltd., Minato, Wakayama 640. System Planning Department, Wakayama Steel Works, Sumitomo Metal Industries, Ltd., Minato, Wakayama 640. Control and OR Technology Department, Instrumentation an and Control Technology Center, Sumitomo Metal Industries, Ltd., Nishinagasu-hondori, Amagasaki 660. Technical Development Department, Sumitomo Special Metals Co., Ltd., Minami Suita, Suita 564.

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the welding temperature and upset amount are vari-ables which must be controlled within very narrow limits during welding. The primary objectives of developing this automatic welding control system were to ensure automatic temperature control, pro-vide the welding operators with a highly accurate temperature profile of the welding zone and con-tinuous data on the upset amount to ensure highly reliable welds, thereby making possible the use of higher strength steels such as high carbon and low alloy steels.

III. Automatic Welding Control System

1. Functions and Features

The outline of these functions is listed in Table 2. The features of the present system are as follows :

(i) The Pattern Thermometer makes it possible to constantly monitor the temperature distribution and the picture at the welding zone and simultane-ously measure and control the temperature at some

point before the welding. (ii) By feed-forwarding variation in wall thickness

and welding speed, it is possible to realize the high controllability of the temperature at the welding zone.

(iii) The upset amount can be monitored con-tinuously.

2. System Structure

Figure 2 shows the structure of the present system. The Pattern Thermometer, Thickness Gauge, Speed Meter and Upset Gauge are installed as sensors for controlling the welding temperature and monitoring. In addition, there is a keyboard/display in the entry section for entering information about skelps and another in the welding section for confirming the information. Two display panels, one in the welding section and the other in the cut-off section indicate passage of the end-welded portions of the skelps.

3. Sensors 1. Pattern Thermometer

(1) Outline The features of the Pattern Thermometer are given below.

( i ) The longitudinal temperature distribution of the welding zone can be observed together with the picture of the welding zone on a color monitor.

(ii ) It generates an analog signal which indicates the temperature at a chosen part within the field of vision of the Pattern Thermometer. Figure 3 shows the structure of the Pattern Thermo-meter. The picture of the welding zone led by an image guide is projected through a rotary filter into a random access camera. The camera output is treated with two color arithmetic operation by micro-computer and indicated as a temperature distribu-tion on a color monitor.

Additionally, the picture of the welding zone, separated by a half mirror, is indicated together with the temperature distribution on the color monitor by means of a color TV camera. Photograph 1 is an example of the color monitor display of the Pattern Thermometer. Photograph 2 illustrates measuring equipment at the welding section. Table 3 gives the specifications of the Pat-tern Thermometer. (2) Temperature Distribution Measurement at

Welding Zone by Pattern Thermometer The functions of the Pattern Thermometer are to measure the temperature distribution at the welding zone and thereby to indicate the temperature dis-tribution as well as the picture of the welding zone on the color monitor display. Figure 4 shows variation in the temperature profiles at various welding speeds. As the speed becomes faster, the gradient of the temperature decreases although the maximum tem-

Table 1. Important imperfection and defects occurring in

High Frequency Electric Resistance Welds.

Table 2. Outline of functions.

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Fig. 2. Equipment.

Fig, 3. Block diagram of Pattern Thermometer .

Table 3. Specifications of pattern thermometer.

Fig. 4. Temperature pattern of welding portion.

Photo. 2. Welder and measuring equipment.

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peratures are almost the same. Photograph 3 shows the welding zone as inspected by the Pattern Thermo-

meter and by a high speed 16 mm camera in 3 levels

of heat input. Thus, the operator can monitor the

welding condition by monitoring the welding tem-

perature for controlling, the temperature distribution and picture of the welding zone on the color monitor

display concurrently.

2. Upset Gauge

(1) Method of Measurement The Upset Gauge can measure the upset amount which is defined as a difference in tube circumference between " before welding " and " after welding ". Figure 5 shows the structure of the Upset Gauge, and Fig. 6 shows the position of sensing heads andPhoto. 1. Color monitor dis play.

Photo. 3. Comparison of weld mg zone ins pected by the Pattern Thermometer and a high speed camera.

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rolls. The measuring method of the Upset Gauge is as follows :

(i) Gaps, GS and Gf, between squeeze rolls and fin pass rolls are measured by distance meters.

(ii) The circumference before welding, lf, and that after welding, ls, are calculated by the following equations:

If = 7 Cf-20f- Tf+2Gf

Is =rCS-2Os+2GS

where, Cs: Radius of squeeze roll caliber Os : Offset of squeeze rolls

GS : Roll gap of squeeze rolls Cf. Radius of fin pass roll caliber Tf . Fin plate thickness of fin pass rolls

of: Offset of fin pass rolls Gf: roll gap of fin pass rolls.

(iii) The upset amount, Us, is a difference be-tween if and ls, namely

Us = if-1s

(3) Accuracy of Measurement To check the accuracy of the gauge, the mill operation was stopped and a comparison was made between the upset value calculated from the gauge and that measured, as shown in Fig. 7. As a result of the development of the Upset Gauge, a high measuring accuracy of ±0.2 mm has been obtained.

4. Automatic Welding Temperature Control 1. Compensation in Heat Input for Changes in Wall

Thickness and Welding Speed A wall thickness gauge and speed meter are neces-

sary for compensating heat input for changes in wall thickness and welding speed. The following equation is generally used to calculate the necessary com-

pensation: Epxlp=KxtaxVa

where, K, a, 8: constants Ep : Plate voltage of oscillation tube

Ip : Plate current of oscillation tube

t: Wall thickness V: Welding speed. As a result of experiments where wall thickness was changed while welding speed was constant, and welding speed was changed gradually while wall thickness was constant, a was calculated to be 1.0 and ~3 to be 0.6. Further, if there is any variation in wall thickness, 4t, voltage compensation required to maintain a constant welding temperature is calculated as follows :

4t = Kt x 4t

where, Kt : constant. Similarly compensation in voltage required for a variation of welding speeds, 4V, is calculated by

4Vt =Kvx4V

where, Kv: constant. The value Kt is classified by wall thickness and out-side diameter and stored in the computer before welding, while KV is classified by wall thickness, out-side diameter and welding speed and stored. Denot-ing the set voltage of the welder at a certain instant by Vi_1, set voltage, Vi, after a lapse of time, 4T, is obtained by

Vi = Vl- +4V1+4V~ .

The effects of speed compensation can be seen in Fig. 8 where the weld quality is monitored together with other weld conditions. The data confirms that the automatic compensation for weld speed is effective in producing a high quality tube even at the time of the mill start when welding conditions change most significantly.

A comparison in weld quality between manual and automatic control shows that automatic tem-

perature compensation for wall thickness deviation is an important factor in maintaining weld tempera-ture within the extremely narrow limits required for welding low alloy steels. The results of this com-

parison are discussed in more detail later.

Fig. 5. Structure of Upset Gauge.

Fig .6. Position of sensing heads and rolls.

Fig. 7. Accuracy of Upset Gauge.

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2, Feed-back Control of Welding Temperature by Pat- tern Thermometer

The Pattern Thermometer allows highly accurate

feed-back control of welding temperature. The field vision of the thermometer is divided longitudinally

into 64 measuring points, numbered 1 to 64 starting from the squeeze rolls side. Figure 9 shows the

relation between heat inputs and measured tempera-tures at points 35, 45 and 55. In the optimum heat

range shown by the shaded area, the line gradient drops, as the measuring point approaches the squeeze

rolls. In addition, the graph shows that there is almost no temperature change in high heat input.

Since a small temperature/heat-input gradient is unsuitable for welding control, measuring point 50 is used as a control measuring point. Included in Fig. 9 are measured examples using the conventional two-color pyrometer. It is impossible to effect minute control of the measuring points when using the two-color pyrometer which measures the temperature within its field of vision as only one point. As can be seen in Fig. 9, the slope of the line within the optimum heat range becomes extremely small, when measurement using the two-color pyrometer is made at a portion 30 mm after the center of squeeze rolls. The better control of welding temperature which is derived from the new Pattern Thermometer is clear.

5. Skelp Tracking

The aims of skelp tracking functions are the fol-lowings; i) To select an optimum welding condition related

with skelp information such as wall thickness and chemical composition.

ii) To reject completely the weld end portion of skelps.

iii) To control products for each skelps. The aforesaid item (ii) is an important function in the high speed mill. Figure 10 shows a general view of an alarm system displaying the positions of the weld end portions. Photograph 4 shows a CRT display of skelp tracking. Owing to this display system, the operator can precisely locate the positions of the weld end portions.

Iv. Weld Quality and Automatic Control

Of various factors affecting the weld quality of high frequency welded tubes, the most influential are the welding temperature and upset amount. These must be kept within very strict limits to ensure that no detects will occur. There are four main weld defects : Excess heat, cold weld, paste weld, and weld penetrator.

1. Welding Conditions and Defects

(1) Excess Heat Figure 11 shows how changes in the welding tem-

perature and upset amount affect weld quality.

Fig. 8. Starting-up

quality.

control and weld

Fig. 9. Characteristic of measurement by Pattern Thermo-

meter.

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Excess heat occurs only when the upset is less than 1.2 mm high and the temperature exceeds 1 300 °C. Figure 12 shows hardly any proper gradient of a metal

flow at the welding portion, and there is a notch along the welded surface extending from the external surface

to the center of the wall thickness. In order to

prevent this defect, it is important to keep a proper

welding temperature matched with the upset amount (see Fig. 16). (2) Cold Weld and Paste Weld

The occurrences of the cold weld and paste weld mainly depend on the welding temperature, and the range where defects occur varies with steel com-

position and tube dimensions. Figures 13 and 14 show the relationship between weld temperature and weld quality of plain carbon steel and low alloy steel tubes, e-valuated by the flattening test. Below the critical temperature, a gray, fractured face is pro-duced after the slight flattening test. Macrostructure analysis also shows that there is no ferrite band in the weld portion. And also, the cold weld occurs at a temperature lower than that for the paste weld. To prevent those defects, it is necessary to control heat input so that the welding temperature will not enter a zone with a temperature below the critical temperature for causing defects. (3) Penetrater and the Like Figure 15 shows how the occurrence of defects in the weld depends on welding temperature. The temperature range which is free from defects is ex-tremely restricted. Conventional welding requires the welding zone to be sealed by an inert gas to prevent the oxidation of elements such as Cr and Mn which causes the penetrater. However, Fig. 15 also shows that automatic control with the new system controls the weld temperature within the critical range allowing welding without sealing.

2. Optimum Welding Range and Automatic Welding Control

Figures 16 and 17 show the schematic diagram of the welding quality and welding conditions for

plain carbon steel (0.12 %) and low alloy (1 % Cr, 0.2 % Mo). As mentioned before, the welding quali-ty is determined mainly by the welding temperature and upset amount. So the operating range must be kept within the very narrow limits required for high-quality tubes. This is especially important for low alloy steel where the range is extremely narrow.

Fig. 10. Diagram of tracking system.

Fig. 11. Upset and welding temperature.

Photo. 4. CRT display of skelp tracking.

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Fig . 12. Macroanalysis of various welding conditions.

Fig, 15, Welding condition and penetrates.

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Without the newly-developed system, it would be difficult to produce high-quality, low alloy welded tubes. The controlled ranges shown are stored in the computer, classified by steel composition and tube dimensions. The optimum conditions are selected according to the operating conditions of the line. An example of a CRT display is shown in Photo. 5, showing the optimum range together with the actual welding conditions indicated by a red spot (indicating on the real-time basis). Figures 18 and 19 compare the welding tempera-tures with manual and automatic control. The

graphs show that under manual control the welding conditions are not stable. Furthermore, the effect Photo. 5. CRT display of welding condition.

Fig. 13. Welding condition and flattening test (1).

Fig. 14. Welding condition and flattening test (2).

Fig. 16. Schematic diagram of welding condition (1). Fig. 17. Schematic diagram of welding condition (2).

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of wall thickness compensation and feed-back control can be seen in the weld portion of coils where the wall

thickness fluctuates. With automatic control, the welding temperature is kept within a range of ±20 °C.

V. Summary

(1) A new automatic welding control system was developed for producing high-frequency ERW tubes and is now operating in No. 2 Tube Mill of the Wakayama Steel Works of Sumitomo Metals. Two new sensors, the Pattern Thermometer and the Upset Gauge were also developed. The system functions include automatic temperature compensations for wall thickness and speed variations, skelp tracking and

production data logging. (2) This system has made possible a constant con-

trol of a stable welding temperature and monitoring of the continuous upset amount, thereby accomplish-ing the production of highly reliable tubes.

(3) This system has been particularly effective for high-grade steels such as low alloy steels and high

carbon steels which have a narrow range

conditions.

of o ptimum

Fig. 18. Manual operation.Fig. 19. Automatic control.

REFERENCES

1) T. Ogawa, H. Kashima, Y. Omotani, Y. Yamaguchi, H. Nakate and M. Azuma : " Recent Production Technique

and Quality of ERW High-Test Line Pipe ", Sumitomo Metals, 31 (1979), 396.

2) F. Arimoto, H. Naruwa, H. Tanaka, A. Takahashi and H. Nakate : " New Ultrasonic Testing Equipments for Elec-

tric-Resistance-Welded Line-Pipe ", Sumitomo Metals, 29

(1977), 469. 3) Y. Higashi, Y. Katayama, T. Katayama, H. Tanaka, A.

Takahashi and K. Yoshimura : " New Rotative Ultrasonic Testing Equipments for Electric Resistance Welded Pipe ", Sumitomo Metals, 30 (1978), 162. 4) N. Torii, H. Nakanishi, K. Kaku and K. Yoshikawa :

" Properties after Long Term Service and High Tempera -

ture Properties on Electric Resistance Welded (ERW) Boiler Tubes ", Sumitomo Metals, 34 (1982), 111. 5) J. Zac and P. Rys: " The Origin of Light Area in Upset

Welds of Rolled Carbon Steel ", Welding Research Suppl.,

(1972), May, 272.

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